Metallic biomaterials with good wear and mechanical properties for implants are commercially available.
These materials can be readily processed to manufacture orthopaedic implants such as hip and knee replacements. Although these implants possess adequate compatibility with human tissue and have a long history of success in joint arthroplasty, there still remains the problem of fixation to the osseous tissue.
A prerequisite for success of any orthopaedic arthroplasty is achieving permanent fixation of the components of the prosthetic device to the surrounding bony environment with no intervening soft tissue. This process, commonly known as osseointegration occurs at the interface between the bone and the implant surfaces. Osseointegration is affected by biomechanical forces and biomaterial properties. The forces transmitted between the prosthesis and the bone depend on the design and geometry of the implant, the materials used, and the mechanical characteristics of the surrounding bone. The biomaterial properties of the surface determine the relative biocompatibility of the material, surface biochemistry, and therefore, the degree of fixation. Among the materials currently in use for implant fixation, commercially pure titanium and hydroxyapatite (HA) most easily achieve osseointegration.
HA and other calcium phosphate coatings have been shown to be effective in clinical applications due to the strong adherent interface formed with the Ti alloy. The most popular method of applying ceramic coatings to metallic substrates is by the plasma spray process. The bond strength of plasma sprayed HA coating on Ti alloy substrates is mainly derived from the combination of mechanical interlocking with the underlying roughened substrate and the chemical bonding that occurs with the TiO.sub.2 layer present on the surface of the substrate. The chemical bonding is believed to occur as a result of the incorporation of Ca.sup.2+ and P.sup.5+ ions into the TiC.sub.x films.
At present, the most commonly used metallic alloy employed to fabricate orthopaedic implants for joint arthoplasty is the ASTM-F75 Co-base alloy Co--27Cr--5Mo--0.3C. Processing of this alloy is carried out by investment casting which presents important economic advantages over the processing techniques employed for implants manufactured from Ti and its alloys.
Fixation of Co-base implants is achieved mainly using polymethyl methacrylate bone cement or by applying a metallic porous coat of Co on the surface of the prosthesis. This coating is applied by sinter-annealing the prosthesis covered with small balls of Co on the surface of interest. In this case, the porosity allows mineralized bone to grow into the porous surface, thus achieving a morphological microinterlock. However, there is no chemical bonding of the metal to the bone, and histologic studies of retrieved porous hip and knee prosthesis have shown variable ingrowth. Hydroxyapatite coating on Co-base alloy prosthesis have not shown the same degree of success as in Ti-base alloy implants.
In Cobalt base alloys, the interdiffusion of P.sup.5+ or (PO.sub.4).sup.3- -ions can occur in the passive coating of Cr.sub.2 O.sub.3. However, the absorption of Ca.sup.2+ necessary for effective biological osseointegration on the implant surface does not occur in these materials. In addition, TiO.sub.x is thermodynamically more stable than the Cr.sub.2 O.sub.3 formed on cobalt base alloys.
On the other hand, the extremely high temperatures involved during the plasma spraying causes the HA to melt in the presence of air. This problem is associated to the structure and composition of the resulting coating. Radin and Ducheyne, reported that plasma sprayed HA coatings retain the basic apatitic structure but were devoid of hydroxyl groups. In addition, several other calcium phosphate phases not present in the starting powder were identified in the plasma sprayed coatings.
These phases include .beta.-tricalcium phosphate(TCP), a-tetracalcium phosphate, and oxihydroxyapatite, as well as non-crystalline calcium phosphate material. Because each calcium phosphate phase has different degree of biocompatibility, it is very important to control the formation of crystalline hydroxyapatite to avoid the total reabsorption of the calcium phosphate coatings into the living tissue.